Gradiometer



March 15, 1960 R, H, vARlAN 2,929,018

GRADIOMETER Filed May 11, 1954 2 Sheets-Sheet 1 #7* EM' I M/XEE fD/scfi/M/A/Aroe 7 '95002052 BY W TTOENEY March 15, 1960 R. H. vARlAN2,929,018

' GRADIOMETER Filed May 11, 1954 2 Sheets-Sheei 2 IN VEN TOR.

GRADIOMETER Russell H. Varian, Cupertino, Calif., assignor to VarianAssociates, San Carlos, Calif., a corporation of California ApplicationMay 11, 1954, Serial No. 429,018

3 Claims. (Cl. 324-.5)

. This invention relates in general to magnetic field measuring methodsand apparatus and more particularly to a novel method and apparatusutilizing gyromagnetic resonance for measuring magnetic field gradientsalong the lines of force of magnetic fields such as the earthls magneticfield.

There are various methods and apparatus presently known for measuringmagnetic fields, one of which is the gyromagnetic resonance methoddisclosed in the U.S. Patent No. 2,561,489, issued to F. Bloch et al. onJuly 24, 1951, and there are other known ways for measuring fieldgradients across magnetic fields. A gyromagnetic resonance method andapparatus for measuring magnetic fields' and gradients is shown inapplicant's copending application entitled Gyromagnetic ResonanceMagnetometer and Gradiometer, filed February 8, 1954, and bearing SerialNo. 408,845. However, in present day applications for magnetic fieldmeasurements such as for geophysical exploration, there has exsted theneed for a method and apparatus for precisely measuring the fieldgraclient along the lines of force of a magnetic field and in additionto measure such gradients in successive lines of field force as aparticular magnetic field is crossed or traversed whereby anomalies inthese gradients caused by ditferences in the physical properties of theearth over its subsurface or foreign objects in the field may bedetected and utilized for obtaining geophysical information or locatingthe foreign objects. The present invention has for its object theprovision ofra novel method and apparatus for measuring magnetic fieldsand field gradients along `the lines of force of a magnetic fields andfield gradients along the lines of force of a magnetic field and formaking readings along successive lines of force in the field Whereby theresults obtained may be utilized for practical applications such as theplotting of contour maps of the gradiations along the lines of force inthe earths field for the purpose'of geophysical survey studies or thelocating of foreign bodies in the field.

One feature of the present invention is the provision of a novel methodand apparatus Whereby the field gradients along lines of force in amagnetic field may be determined.

h 'Another feature of the present invention is the provision of a novelmethod and apparatus utilizing'gyromagnetic resonance for measuring thestrength of a magnetic field.

Still another feature of the present invention is the provision of anovelmethod and apparatus whereby field g'radients along successivelines of force in a magnetic field may be determined and may be plottedto give contour maps and the like which are highly useful in geophysicalexploration. i

Still another feature of the present invention is the provision of anovel method and means for'm'easuring 2,2%,018 Patented VMar.V 15, 1960methods of field measurement such as magnetic airborne detectors.

Another feature of the present invention is the reduction or eliminationof the effects of naturally occurring fluctuations of the earth'smagnetic field in the gradiometer readings. i i

Still another feature is the elimination of the requirement present inother known field measuring methods for maintaining a level alttudeduring airborne magnetic surveying.

Still another feature of the present invention is the accentuation ofanomalies produced relatively close to the apparatus relative to distantor deep anomalies.

These and other features and advantages of the present invention willbecome apparent from a perusal of the following specification taken inconjunction with the accompanying drawings wherein Fig. l discloses oneembodiment of the novel gyromagnetic resonance apparatus which isutilized in performing the field gradient measurements,

Fig. 2 is a diagrammatic illustration of how the present invention maybe utilized in measuring the field gradients along the lines of force ofthe earth's field from a rapidly moving airplane, successive lines offorce being measured.

Referring now to Fig. 1 there is shown two similar pieces of apparatus 1and 2, each located at a different point along a line of force 3 of amagnetic field which,` for purposes of illustration, shall be taken tobe the earth*s field, the force line extending from the earth's surfaceas indicated by the arrow on line 3. Since the two apparatus areidentical, at least in the preferred embodiment of this invention, onlyone will be described in detail. The apparatus 1 comprises a permanentmagnet device 1' having three magnetic poles 4, 5 and 6 and two air gaps7 and 8, pole 6 serving as a common pole to the gaps ,7 and 8. The polepieces 4, 5 and 6 are preferably made of material having a very highpermeability such as an alloy iron, for example, permalloy, or a highpermeability ferrite such that small magnetizing force changes willproduce relatively large flux density changes in the gaps. The yoke 9 ofthe magnet device is preferably of a magnetic material having a highresidual magnetism relative to the magnetizing flux deni' sity, that is,one having a relatively square hysteresis'loop. This is desirable sothata changing magnetizing ,force on the magnet device will result inlittle flux. density change in the yoke, the fiux density changes thusbeing concentrated through the pole pieces 4, 5 and 6. v I i This magnetdevice 1' produces a magnetic field across the twogaps 7 and 8, thefield in gap 7 extendng downwardly and the 'field in gap 8 extendingupwardly as viewed in Fig. 1. As above stated, this magnet apparatus is`axially aligned in an external magnetic field line of force, such asthe earth's, and this force is directed through the pole pieces 5, 6 and4, thus producing an additional field across the gaps 7 and 8directedupwardly in both cases, this field adding to the magnet field ingap` 8 and subtracting from the magnet field -in gap 7.?v The fieldstrength between the gaps due to the earth's field may be as high as onehundred times the earth's due to the intensifying properties of themagnet. iIt should be understood that the particular magnet deviceil',shown is only for illustration purposes. and that ('Stliei''` struoturessuch as, for example, an electromagnet could be employed in lieu of thepermanent magnet device 1', in which case identical solenoids carryingtheV same D.C. current would be found around eachA of the fourfly'okeSectionsV or identical coils With no iron could be fused;

Positioned in each of these gaps 7 and 8 is4 one wwe-` guide sidearm11'and 12 of a pair of magic Ts or hybrid' junctions 13 and 14. Each ofthese magic T 's forms a.

zasadom part of the feedback circuit of an associated oscillator 15 and,16 of a type such as that disclosed in a U.S. patent applicationbearing Serial No. 367,538, filcd July 13, 1953, by Marvin Chodorow andRussell H. Varian entitled Gyromagnetic Resonance Apparatusl Oneof theseoscillators will be described, the upper one 15, and itis to beunderstood that the other oscillatoriis identical.

Positioned in the waveguide side arm 1,1 within the associated field gap7 is a small volume 17 of a gyromagnetic substance of the paramagneticclass such as alkali metal in ammonia or diphenyl-picryl-hydrazyl. Theother waveguide side arm 18 of the magic T 13 is a dummy load which isused to balance the magic T before gyromagnetic resonance occurs in theother side armll. With noise present at the Larmor frequency of thegyromagnetic substance in the side arm 11, gyromagnetic resonance occursin side arm 11 resulting in an unbalancing of the magic T. When themagic T 13 is unbalanced, a portion of the output from the associatedmicrowave amplifier 19, which, for example, vmay be a two cavityklystron, is transmitted through one arm 21 of the magic T and throughthe other arm 22 of the magic T back to the amplifier input to therebyprovide a feedback circuit to the amplifier which Will then oscillate.The frequency at which self-oscillations will occur is deterrnined bythe strength of the magnetic field across the gap 7, this strcngthdetermining the frequency of gyromagnetic resonance of the electrons intheparamagnctic substance 17, and this resonance frequency in turndetermining the oscillation frequency of the oscillator. The frequencyof oscillation is thus determined .by the resultant field due to themagnet's field and the field induced across the gap due to the earthlsfield, assuming the earth's field to be under test. The detailedexplanation of the operation of this oscillator is set forth in theabove cited patent application Serial No. 367,538. The lower oscillator16 operates in a similar manner to oscillator 15, the frequency ofoscillation being determined by the field strength across gap 8.

The radio frequency output l from oscillator 15 will 'be different thanthe radio frequency output 2 from oscillator 16 since the resultantmagnetic vfield across gap 8 is Stronger than the resultant magneticfield across gap 7. The frequency difference between vthe ,frequenciesof signals l and 2 will be Proportional to twice the earth's field atthe midpoint between gap 7 and 8 since the earthfs fifild iS added tothe magnet field in one gap and subtracted in the other. The differencein the frequency output of the two oscillators 15 and 16 may be easilyobtained by supplying the output from each of these oscillators to amixer 2,3 where the two radio frequencyvsignals fg and l are ubeat," thevalue of the frequency of the difference or beat frequency signal faobtained being proportional to the strength of the earth*s magneticfield at the midpoint between the gaps 7 and 8.

To give a concrete example, the gyromagnetic resonance frequency ofelectrons in a typical paramagnetic substance is 3.6)(106 cycles persecond per gauss. Since the earth's field isapproximately V2 gauss andsince the ferromagnetic core increases it by, say, a factor of 100, thegyromagnetic resonance frequency in each paramagnetic substance in thearms 11 and 12 due to the earth?s field is 3.6 106x50. Since thegyromagnetic frequency in each substance due to the field produced bythe magnet cancel out in the mixer 23, the difference frequency of theoutput fa from the mixer is twice the gyromagnetic resonance frequencyin each arm due to the field induced therein by the earthis field. Thus,in this illustration, the output frequency from the mixer 23 will be 3.650j 2 or 3.6)(108 cycles/sec.

It should 'be noted that the results are given in terms Qfpvfrequencyand changes in frequency which are a great deal more accurate andreliable than readings in current, VOllage and 'the like. It should alsobe noted that the difference frequency will vary only as a `.Secondorder 4 function of the departure of the magnet structure 1' from directalignment in the line of force 3 of the field.

The lower apparatus 2 operates in a manner similar to the apparatus 1,the magnet device 24 being located in axial alignment With the magnetdevice 1' in the line of force of the earth`s field, the two devices 1'and 24 being positioned a selected distance apart, for example two feet.The apparatus 1' and 24 are 'preferably fixedly mounted on a common baseor framework 10 so that the relative spacing and exact alignment will bemaintained.

In this embodiment the magnetic field across gaps 25 and 26 due to themagnet 24 is the same as the magnetic field across gaps 7 and S producedby the magnet 1' above. A suitable fine-adjusting metal member 20 isslidably mounted on the magnet 24 so that the magnetic fields acrossgaps 26 and `25 may be changed slightly if desred to bring them intobalance with the fields of magnet 1'. The earth`s field strength at themidpoint between gaps `7 and 8 is different from the earths fieldstrength at the midpoint 'between the gaps 25 and 26 since the strengthof the earth's field decreases With increasing distance from the earth.

Thus, the fieldl strength due to the eartlrs field at the midpointbetween gaps 25 and 26 is greater than ,the field strength due to theearth's field at the midpoint betweenv gaps 7 and 8. The field in gap 25due to the earth's field subtracts from the magnet's field in gap 25 andadds to the magnet's field in gap 26. Since the earth's field is largerat magnet apparatus 24, the resultant field across gap 26 is larger thanthe resultant fields across either gaps 7 or l8 while the resultant,field across gap 2,5 is smaller than theresultant fields across gaps 7or 8, Thus, the frequency of signal f4 Of oscillator 27 isless than thefrequency of signals l or z while the frequency of signal s ofoscillator 28 is greater than the frequency ofsignals l or z. Thefrequency of difference signal fi between the signals 4 and ;f asobtained from the mixer 29 is thus greater than the frequency obtainedfrom the mixer 23. The signals fa and are transmitted to another mixer31 where the two signals are beat together to give a differencefrequency output signal q.

The difference frequency of signal is a measure of the differencebetween the strength of the earths magnetic field at the midpointbetween gaps 7 and 8 and the strength of the earthfs magnetic field atthe midpoint between the gaps 25 and 26. V

VReturniug to the example above, the earth's field at device 1'v wasassumed to be 1/2 gauss. The earth's field at the midpoint between gaps25 and 26 is then 1/2 gauss plus a certain amount whichis dependent onthe gradient along the line of force. .A representative field gradientwould be about 50 gammas per mile, one gamma equalling 10-5 gauss. Sincethe distance between the midpoints is 2 feet, the gradient along the twofeet is roughly 2x10-2 gammas or 2X10-7 gauss. The earth's field betweengap 25 and 26 is thus 1/2 gauss plus 2 10-'l gauss. Following throughwith the same reasoning as above in arriving at the output frequency ofmixer 23, the output frequency from the mixer 29 will be 3.6 10B+144cycles/sec. Thus the difference frequency of signal q is 144 cycles/sec.

The output signal Vfrom the vmixer 31 may be trans' mitted to Vastandard type RM. discriminator 32 which will give a D.C. voltageoutput,the magnitude of which will vbe proportional to the frequency of thesignal q. This D.C. voltage output may then be transmitted to a suitablerecorder 33 for permanent recording. This is only one illustration of away for recording the results. Many other ways will be evident to thoseskilled in the electronics art.

' .The output signal obtained when the magnet apparatus 1' and 24 arealigned in the line of force '3 may be equal to or difierent from thefrequency of `the .output signal obtained when the magnet apparatus ismoved :to aligfimcnt .in another line o'f .force of the vearthls spadniefield,I depending on whether or not the difference or gradient in fieldstrength between equally spaced points in the two lines of force isequal. Therefore, anomalies m the field ditferences between equallyspaced points in different lines of force will be indicated bydilferences m the frequency of the output signals and, since theseanomalies are due in major part to the differences in electricalconductivity and magnetic susceptibility in the rock formations and thelike beneath the respective force lines, there is provided a valuabletool in geophysical prospecting. Assume that the magnet devices 1' and24 are aligned m a line of force different from line 3, the distancebetween devices 1' and 24 remaining the same. Assume also' that thefield strength at the midpoint between gaps 25 and 26 is the same in thenew line' of force as in line 3, but that the field gradient in the newline is greater than the field gradient in line 3. That is' to say, thefield strength at the midpoint between gaps 7 and 8 in the new line offorce is less than the field strength at the like point in line 3. Underthese conditions, the output signals fa, and will be of the samefrequency as when the apparatus 24 was in line of force 3 but thefrequencies of outputs l and 2 will be different for the followingreason. The resultant field strength across gap 8 will be less in thenew line of force than in line 3 while the resultant field strengthacross gap 7 will be greater since there is less earth's field `to addin gap 8 and oppose in gap 7. Therefore, the frequency of 1 will behigher than previously while the frequency of ;fg will be less thanpreviously. The difference frequency fa will therefo're be less thanwhen the apparatus was positioned in line of force 3. Following fromthis, the difference frequency of 7 will be greater with the equipmentin the new force line than in line 3. It can thus be seen thatincreasing frequency outputs indcate increasing field gradients and viceversa.

Returning to the illustration, assume the gradient in the new line offorce is 40 gammas per mile as dstinguished from the 50 gammas per mileassumed in force line 3. The gradient across the two feet in thisinstance would be roughly 1.6 10-7 gauss. From this it can be calculatedthat the output frequency from the mixer 29 will be 3.6 106 (1z+1.610-'7) 100 2 which equals 3.6X108+115 cycles/sec. Thus the differencefrequency is 115 cycles per second. Thus, as this apparatus includingthe magnet devices 1' and 24 is moved through subsequent lines of force,a D.C. voltage recording may be obtained which will be a measure of thedifference in field gradients along the lines of force and, if the pathof the movements is no'ted and recorded in any of the known manners, acontour map revealing these varying gradients along the lines of forcemay be plotted.

Fig. 2 is a diagrammatic illustration showing the magnet devices 1' and24 of a gradiometer of the type shown in Fig. 1 postio'ned in anairplane 34 so that they are aligned with the lines of force of theearth's magnetic field. The recorded data may then be utilized inplotting a contour map of the terrain covered showing the variations inmagnetic field gradients and thus Spotlighting the magnet anomalities inthe earth's field.

It should be noted that one of the main advantages of measuring fieldgradients along lines of force of the earth's magnetic field lies in thefact that although the magnetic field difference or gradient between twopoints may be small compared to the actual field at either point, andtherefore a mo're Sensitive instrument is required to measure gradients,the ratio of that portion of the gradient produced between the pointsdue to an anomaly in the earth's field to that portion of the gradientdue to the earths field itself is relatively large. This is due to thefact that the field (F) produced by a magnetic dipole is proportional towhere r is the distance between poles while the gradient dF i isproportional to and since the effective distance of the earth's dipoleis some thousands of times farther away from the gradiometer apparatusthan the dipole producng the anomaly in the field, the effect of theanomaly is some thousands of times greater relatively when the gradientsare-compared than when the fields themselvesiare compared. Because ofthis, the variations in the altitude -of the' airplane as thegradiometer readings are made is' ofpractically no consequence. Also,stabilizatio'n, that is alignment of the magnet devices 1' and 24 in thelines of force, varies only as a second order function of the departurefrom alignment and in the present invention is only a factor of o'neone-hundredth (171100) as critical as stabilization in presently usedmagnetic airbome detectors. Also, fluctuatons in the earths field suchas secular variations, solar diurnal variations, lunarl diurnalvariations, and the more rapid fluctuatons or naturally occurringchanges will no't noticeably ai'fect the gradiometer readings since theeffects due to the changes in the earths field at the two points will besubstantially canceled out.

Since various modifications may be made in the described method andapparatus without departing from the spirit of the invention, such as,for example, the use of an arrangement o'f field controlled bridges inlieu of the oscillator equipment shown in Fig. 1, it is intended thatthe foregoing description is to be considered as exemplary and not in alimiting sense.

What is claimed is:

1. In combination, means for producing a 'first pair of mutually alignedmagnetic fields having equal intensities, said fields extending inopposite directions such that when said means is placed in a thirdmagnetic field substantially aligned with said pair of fields andextending in one of said directions, one of said pair of magnetc fieldsis increased proportionally to said third field while the other of saidpair of magnetic fields is decreased, two' volumes of matter havingportions of atoms having the properties of magnetic moment andgyroscopic moment, each volume being within a different one of said pairof aligned magnetic fields, means for producing a second pair ofmutually aligned magnetic fields having equal intensities, said secondpair of fields extending in opposite directions such that when saidmeans is placed in said third magnetic field such that said second pairof fields is substantially alignedA with said third magnetic field andsaid first pair of magnetic fields one of said second pair of magneticfields is increased proportionally to said third field while the otherof said second pair of magnetic fields is decreased, two o'ther volumesof matter having portions of atoms having the properties of magneticmoment and gyroscopic moment, each of said latter two volumes beingwithin a different one o'f said second pair of aligned magnetic fields,means for producing Larmor precession of the atom portions in each ofthe four volumes o'f matter, the strength of the respective magneticfields determining the frequency of precession of the portions of atomsin each volume, and means associated with each of said four volumes ofmatter fo'r producing radio frequency output signals proportional to thefrequencies of precession of the gyromagnetic portions of atoms in eachvolume of matter.

2. A combination as claimed in claim 1 wherein said means for pro'ducingsaid radio frequency signals cout' prses four radio frequencyoscillators coupled to separate ones of said volumes of matter wherebythe oscillating frequency of each oscillator is dctermined by thefrequency of precession of the gyromagnetic portions of atoms'in its'associated volume of matter.

3. Apparatus for determining field gradients along a line of force of amagnetic field which comprises means for Iocating at a first point'inthe line of force a fi-rst and second volume of matter containingportions of atoms having gyrornagnetic properties aligned in the line offorce and positioned close to each other, means for applyingasecondmagnetic field to one of said volumes adding to the -first fieidto thereby produce a first resultant field Vand for applying a thirdmagnetic field equ-al to said second magnetic field to the other of saidvolumes opposed to `the lfirst magnetic field to thereby produce asecond resultant field, means for producing gyromagnetic reso-` nance ofthe atom portions in each volume, the frequency of lresonance -in eachvolume being determined by the strength of the respective resuitantfields, the difference in resonance frequencies being a measure of thestrength ofthe first field at that point in the line of force, means forlocating a third and fourth volume of matter containing `portions of'atorns having gyromagnetic properties at a point a known distance fromthe point of the other two volumes of matter aligned in the line offorce and positioned Aclose .to each other, means for applying a fourthmagnetic field to saidithird volume adding to the first 'field to:there'by `produfce 'a 'third 'resultant field and' forapplying.fa"fifth magneticfield eqttal to said fourth magnetic field to said fourth'volume opposing the first magnetic field to thereby prodnce a fourthresultant field, and means for producing gyromagnetic resonance of theatom portions in each said third and fourth ivolumes, the frequency ofresonance in each volume being determined by the strength of therespective resultant fieids, the difference in resonance frequenciesbeing a measure of the strength of the -first field at the respectivepoint in the line of force, the' difierences in the strength at the twopoints serving to indicate the field :gradient along the line of force.

References Cited in the file of this patent UNITED STATES PATENT S2,382,743 Penther et al. Aug. 14, 1945 2.;500,186 Kline Man 14, 19502,520,677 Fearon Aug. 29, 1950 `2,561,49() Varian July 24, 1951 2,620381Mayes et al Dec. 2, 1952 2542,479 Jones June 16, 1953 2,'663,843Wickerham et al Dec. 22, .1-953 2,-671,27'5 'Burns Mar. 9, 1'954'2,-693,,590 Schmitt Nov. 2, 1954 V2,`709;783 Hate May 3`1, 1955'

